1. Field of the Invention
The present invention relates to an improved Si-containing magnesium alloy for pressure casting or gravity casting with a melt of the alloy.
2. Description of the Related Art
In a wide sense, a method of casting an alloy includes casting with a melt of the alloy and casting with rapidly solidified particles or the like of the alloy. The melt alloy casting method includes pressure casting such as high or low pressure die casting and squeeze casting. The rapidly solidified alloy casting comprises the steps of rapidly solidifying a melt of the alloy to produce alloy particles or the like, and consolidating the particles by compacting.
With respect to the rapidly solidified alloy casting method, it has been recognized that there is no serious problem in castability, particularly hot cracking, since the two above mentioned steps in combination prevent such hot cracking from occurring, whereas the melt alloy casting inherently encounters such a problem, since the melt is solidified in a single casting step to form a cast product.
One of the known typical magnesium alloys for casting is AZ91, and it has been recognized as having superior mechanical properties and castability when it is used for gravity casting, particularly AZ91 is known as a magnesium alloy having a system of aluminum and zinc exhibiting less hot crack sensitivity in the gravity casting.
For pressure casting, such a Mg--Al--Zn system alloy as AZ91 has been modified by adding Si to diminish or reduce hot cracking and also improve creep strength. However, in the case of a die cast or squeeze cast product with a large variation in thickness, the addition of Si cannot prevent substantial occurrence of hot cracking, particularly in the case of the squeeze cast product.
Further, it is noted that a Si-containing cast product having a portion where a cooling rate during solidification is relatively low has coarse granular eutectic compounds of Mg.sub.2 Si produced, which have a detrimental effect on mechanical properties of the cast product.
In connection with the above, it is noted that JP-A63-220961 discloses a Si-containing magnesium alloy for die casting consisting of 3.7-4.8 wt % of Al, 0.22-0.48 wt % of Mm, 0.69 to 1.4 wt % of Si and the balance of Mg and impurities, wherein the addition of Si is recognized as reducing gas porosity and exterior shrinkage porosity.
JP-A63-220962 discloses a Si-containing magnesium alloy for porosity-free casting of a product having a complicated profile, at a die temperature of 100.degree. to 150.degree. C. The alloy has the same composition as that of JP-A63-220961. The reference states that Si improves a creep property of the cast alloy at a high temperature and prevents occurrence of cast cracking, and it is necessary to add not less than 0.69% of Si to obtain these effects, while over 1.4% of Si is prone to hot cracking as well as exterior shrinkage.
U.S. Pat. No. 5,147,603 relates to the above mentioned rapidly solidified alloy casting, and discloses a Si-containing magnesium alloy consisting, by weight %, of 2-11% of Al,0-1% of Mn, and 0.1-6% of Sr with the following content of the main impurities: Si&lt;0.6%; Cu&lt;0.2%; Fe&lt;0.1%; and Ni&lt;0.01%, the remainder being Mg. It is noted in the reference that Si may exist as an impurity. This means that Si is not an essential element to be added. Sr is added to improve mechanical properties of alloy, particularly to obtain a high breaking strength or high load at rupture exceeding 400 MPa.
A report given at the 96th AFS Casting Congress, entitled "Effect of Strontium on the Shrinkage Microporosity in Magnesium Sand Castings", by C. A. Aliravci, J. E. Gruzleski, F. C. Dimayuga, May 3-7, 1992 (American Foundations Society Inc.) states, in conclusion, that: Sr has a strong effect on the distribution of shrinkage microporosity of AZ91C alloy castings; additions of up to 0.02% of Sr trend to concentrate shrinkage microporosity at the hottest section while minimizing it in the rest of the castings; the optimum level of Sr addition that promotes this effect was found to be between 0.01% and 0.02% of Sr, whereas with additions made both above and below this range the effect rapidly disappears; thermal analysis showed that the addition and dissolution of Sr alters the grain size in AZ91C alloy melt; and the SEM-based grain size analysis technique verified that the addition of 0.01% to 0.02% of Sr produces a fine grain size of 120 .mu.m while castings with no Sr have a coarser grain size of 250 .mu.m.
In connection with the above report, it should be noted that AZ91C consists of, by weight %, 8.1-9.3% of Al, &gt;0.13% of Mn, 0.4-1.0% of Zn, &lt;0.30% of Si, &lt;0.10% of Cu, &lt;0.01% of Ni, and the balance of Mg with impurities.
Another report given at the Magnesium International Congress, 1992 by B. L. Mordike and F. Hehmann Editors, entitled "Magnesium Alloys and Their Applications" states, in conclusion, that: addition of Sr to AZ91 magnesium alloy, and probably to all Mg-Al based conventional cast alloys, results in a better castability, but the amount of Sr necessary to produce such changes seems to depend upon the casting conditions; in the casting conditions used in this work, cast parts of AZ91+0.3% Sr with considerably reduced microporosity were obtained, without significant loss in strength or ductility at room temperature; the presence of Sr at these levels also seems to induce better creep properties and improved performance in accelerated corrosion tests; and an explanation for these observed changes in alloy performance has been proposed in terms of the microstructure changes in the presence of Sr: grain refinement and new Sr containing needle-shaped precipitates. However, the present inventor has found from the data given in the above report that the presence of Sr does not seem to induce better creep properties contrary to the above conclusion.
With respect to the Mg--Al--Zn ternary system cast alloys, there are two reports to be noted, entitled "Investigation Regarding Magnesium Die Cast" by G. S. Foerster, Dr. P. C. J. Gallagher, D. L. Hawke and Dr. E. N. Aqno (Die Casting Engineer, 1-2, 1977), and "Properties of Mg--Al--Zn Ternary Cast Alloys" by H. Ishimaru, J. Kaneko and M. Sugamata (the 58th Spring Congress of Light Metal Academy in Japan, 1980).
The first report (Foerster et al) illustratively teaches in a binary Al--Zn system diagram of the magnesium cast alloy (FIG. 2) that: hot cracking does not occur and thus the alloy is castable in a zone having less than about 1.5 wt % of Zn, irrespective of the content of Al; the alloy is castable, for example, in a zone having more than about 6.0 wt % of Zn and more than about 2.5 wt % of A1; the alloy is castable, for example, in a zone having more than about 4.0 wt % of Zn and more than about 6.0 wt % of A1; and the alloy is not castable in at least a zone having less than about 5.0 wt % of Zn and less than about 4.0 wt % of Al .
The second report (Ishimaru et al) also illustratively teaches in a corresponding binary Al--Zn system diagram of the magnesium cast alloy (FIG. 6) that: hot cracking does not occur in a zone having less than about 2.0 wt % of Zn, irrespective of the content of A1; it does not occur, for example, in a zone having more than about 6.0 wt % of Zn and more than about 8.0 wt % of Al; it does not occur, for example, in a zone having more than about 8.0 wt % of Zn and more than about 6.0 wt % of Al; and it occurs in at least a zone having 2.0 to 6.0 wt % of Zn and less than 8.0 wt % of Al.